Introduction to Organic Chemistry

Edexcel International GCSE Chemistry· 4.1–4.6 (sub-topic 4(a))· 15 min read

1. Hydrocarbons and Organic Compound Basics★☆☆☆☆⏱ 2 min

Organic chemistry is the study of carbon-containing compounds, excluding simple inorganic carbon compounds like carbon dioxide and carbonates. The simplest group of organic compounds are hydrocarbons.

📘 Definition

Hydrocarbon

A compound that contains hydrogen and carbon atoms only, with no other elements present.

Example:

Methane (CH₄) is a hydrocarbon; ethanol (C₂H₅OH) is not, as it contains oxygen.

📐 Worked Example

State whether each of the following is a hydrocarbon: (a) C₃H₈, (b) C₂H₄O₂, (c) C₆H₁₄

1
Check if the compound only contains carbon (C) and hydrogen (H) atoms, with no other elements.
2
(a) C₃H₈ contains only C and H: it is a hydrocarbon.
3
(b) C₂H₄O₂ contains oxygen (O): it is not a hydrocarbon.
4
(c) C₆H₁₄ contains only C and H: it is a hydrocarbon.

Exam tip:

When asked to identify hydrocarbons, always check for the presence of other elements like oxygen, nitrogen or halogens before answering.

2. Organic Compound Formula Types★★☆☆☆⏱ 4 min

🚫 No Calculator

There are 5 key formula types you need to be able to use and interpret for organic compounds, as defined by the specification:

Empirical formula: The simplest whole number ratio of atoms of each element in a compound
Molecular formula: The actual number of atoms of each element in one molecule of the compound
General formula: A formula that applies to all members of a homologous series, using $n$ to represent the number of carbon atoms (e.g. $C_{n} H_{2 n + 2}$ for alkanes)
Structural (condensed) formula: A formula that groups atoms bonded to each carbon, without showing all bonds (e.g. $C H_{3} C H_{2} C H_{3}$ for propane)
Displayed formula: A 2D representation that shows every atom and every covalent bond in the molecule

📐 Worked Example

Give the empirical, molecular, structural and displayed formula for ethane, which has 2 carbon atoms and 6 hydrogen atoms.

1
Empirical formula: Simplify the ratio of C:H from 2:6 to 1:3 → $C H_{3}$
2
Molecular formula: Exact count of atoms → $C_{2} H_{6}$
3
Structural formula: Condense bonded atom groups → $C H_{3} C H_{3}$
4
Displayed formula: Show all atoms and bonds:
5
$\ln \chemfig{H-C(-[2]H)(-[6]H)-C(-[2]H)(-[6]H)-H}\ln$

Exam tip:

Full marks for displayed formula are only awarded if every single bond, including all C-H bonds, are drawn explicitly. Do not omit any bonds when asked for a displayed formula.

3. Homologous Series, Functional Groups and Isomerism★★★☆☆⏱ 4 min

Three core terms define how we group and distinguish organic compounds: homologous series, functional group and isomerism.

📘 Definition

Homologous Series

A family of organic compounds that share the same general formula, same functional group, similar chemical properties, show a gradual trend in physical properties (e.g. boiling point increases with carbon count), and differ from each adjacent member by a $- C H_{2} -$ group.

Example:

The alkane series (methane, ethane, propane, etc.) is a homologous series.

A functional group is an atom or group of atoms that determines the characteristic chemical properties of a compound. For example, all alkenes have a C=C double bond functional group, which makes them react in similar ways.

Isomerism describes compounds that have the same molecular formula but different structural arrangements of atoms. At IGCSE level, we only consider structural isomerism; no other forms of isomerism are required.

📐 Worked Example

Explain why butane ( $C_{4} H_{10}$ ) and methylpropane ( $C_{4} H_{10}$ ) are isomers.

1
First confirm both compounds have the same molecular formula: $C_{4} H_{10}$ for both.
2
Check the structural arrangement: Butane has a straight 4-carbon chain, methylpropane has a 3-carbon chain with a methyl branch attached to the second carbon.
3
Since they have the same molecular formula but different structures, they are structural isomers.

Exam tip:

When asked to explain isomerism, always explicitly state that the compounds have the same molecular formula and different structural formulae to get full marks.

4. Basic IUPAC Naming Rules★★☆☆☆⏱ 3 min

You need to be able to name simple organic compounds with up to 6 carbon atoms using IUPAC rules. The name is made of two parts: a stem that tells you the number of carbon atoms in the longest chain, and a suffix that tells you the functional group present.

Number of carbon atoms	Stem name
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

The suffix depends on the functional group: -ane for alkanes (only single C-C bonds), -ene for alkenes (C=C double bond), -ol for alcohols (-OH group), -oic acid for carboxylic acids (-COOH group).

📐 Worked Example

Name the compound with a 3 carbon chain and only single covalent bonds between carbon atoms.

1
Identify the stem for 3 carbon atoms: prop-
2
Identify the suffix for only single C-C bonds: -ane
3
Combine the stem and suffix to get the name: propane

Exam tip:

Double-check the number of carbon atoms to match the correct stem; mixing up prop- (3C) and but- (4C) is a common exam error.

5. Organic Reaction Types★★☆☆☆⏱ 2 min

You only need to classify organic reactions into three categories for this topic: combustion, addition and substitution. No reaction mechanisms are required.

Combustion: Reaction of an organic compound with oxygen, producing carbon dioxide and water (complete combustion) or carbon monoxide/carbon (incomplete combustion), releasing heat energy.
Addition: Reaction where two molecules combine to form one single product, with no other products formed. Happens to compounds with double bonds, e.g. alkenes.
Substitution: Reaction where an atom or group of atoms in a molecule is replaced by a different atom or group of atoms, forming two products.

📐 Worked Example

Classify the reaction of ethene ( $C_{2} H_{4}$ ) with bromine ( $B r_{2}$ ) to form only 1,2-dibromoethane ( $C_{2} H_{4} B r_{2}$ ).

1
Check the number of reactants and products: 2 reactants form 1 single product.
2
This matches the definition of an addition reaction.

Exam tip:

If the reaction has only one product, it is almost always addition; if one atom is swapped for another, it is substitution; if oxygen is a reactant and oxides are products, it is combustion.

6. Common Pitfalls

Wrong move:

Calling a compound containing oxygen a hydrocarbon

Why:

Hydrocarbons are strictly defined as compounds containing only carbon and hydrogen, with no other elements.

Correct move:

Always check for elements other than C and H before identifying a compound as a hydrocarbon.

Wrong move:

Omitting C-H bonds when drawing displayed formulae

Why:

Displayed formula requires all atoms and all covalent bonds to be shown explicitly to get full marks.

Correct move:

Draw every single covalent bond, including all bonds between carbon and hydrogen atoms, for displayed formula questions.

Wrong move:

Defining isomerism as 'same general formula, different structure'

Why:

Isomers have the same molecular formula, not just the same general formula (which applies to all members of a homologous series).

Correct move:

Always specify that isomers share an identical molecular formula but have different structural arrangements.

Wrong move:

Using the wrong stem for carbon count, e.g. using but- for 3 carbon atoms

Why:

Stem names are fixed by the number of carbon atoms in the longest unbroken chain of the compound.

Correct move:

Memorize the 6 stems (meth to hex) and match them correctly to carbon counts 1 to 6.

Wrong move:

Confusing addition and substitution reactions

Why:

Addition reactions produce only one product, while substitution reactions produce two products.

Correct move:

Count the number of products: 1 product = addition, 2 products = substitution.

7. Quick Reference Cheatsheet

Concept	Key Exam Details
Hydrocarbon	Only contains C and H atoms, no other elements
Formula types	Empirical (simplest ratio), Molecular (actual count), General ( $C_{n} H_{x}$ ), Structural (condensed), Displayed (all bonds shown)
Homologous series	Same general formula, same functional group, differ by $C H_{2}$ unit, gradual physical property trend
Isomerism	Same molecular formula, different structural arrangement of atoms
Naming stems (1-6C)	meth, eth, prop, but, pent, hex
Reaction types	Combustion (O₂ reactant), Addition (1 product), Substitution (swap atom/group)

8. Frequently Asked

Do I need to learn reaction mechanisms for this topic?

No, the Edexcel IGCSE 4CH1 specification explicitly states reaction mechanisms are not required for this sub-topic. You only need to classify reactions as substitution, addition or combustion.

What is required for a displayed formula to get full marks?

A displayed formula must show every single atom and every covalent bond in the molecule, including all C-H single bonds. Omitting any bonds will lead to lost marks.

Going deeper

official_syllabusEdexcel IGCSE Chemistry 4CH1 2017 Specification

What's Next

Now that you have mastered the foundational concepts of organic chemistry for Edexcel IGCSE Chemistry, you are ready to move on to studying specific homologous series. The next core topics cover alkanes, the simplest saturated hydrocarbons, followed by alkenes, the unsaturated hydrocarbons with C=C double bonds. These topics build directly on the formula writing, nomenclature and reaction classification skills you have learned in this introduction. You will also later apply these basics to chemistry-only topics including alcohols, carboxylic acids and esters if you are studying the full Chemistry IGCSE. Practice drawing formulae and naming compounds regularly to build confidence before moving to more complex organic reactions.